| Zeolite membranes are widely used in pervaporation because of their good mechanical strength,chemical stability and uniform pore structure.The recent years have witnessed the fast development of pervaporation technology.Since NaA zeolite membranes were industrialized by Mitsui Engineering&Shipbuilding Co.Ltd.in 1998,there are also some institutions making joint efforts to industrialize the NaA zeolite membranes in China.However,the application of NaA membranes in acidic solution is limited for it has a low Si/Al ratio of 1.T-type zeolite(OFF-ERI intergrowth)with effective pore size of 0.36 nm×0.51 nm and Si/Al ratio of 3-4,is a suitable candidate for the dehydration in acidic condition and gas separation.Conventional hydrothermal synthesis of T-type zeolite membranes generally needs more than 30 h,which is longer than most of zeolite membranes.Moreover,secondary growth method is usually used.The low flux of T-type zeolite membranes on tublar supports restricts its further application in industrial production.Therefore,it is necessary to explore a more rapid,simple and energy-saving preparation method.In order to solve the above problems,we choose ?-Al2O3 hollow fibers as supports to improve the flux of T-type zeolite membranes;we use ultrasonic-assisted aging process to reduce the reaction time of hydrothermal synthesis and prepare T-type zeolite membranes with high separation performance;At the same time,the synthesis of T-type zeolite membranes by hot water seeding on dopamine hydrochloride(DA)modified supports is also studied.The seeding technique is simplified,and its mechanism is analysed.(1)T-type zeolite membranes synthesized by ultrasonic-assisted aging processT-type zeolite membranes are prepared on alumina hollow fibers in different ultrasonic treating times and different synthesis times.Scanning electron microscopy(SEM),X-ray diffraction(XRD),pervaporation(PV)and other methods are used to characterize the membrane properties,powder in synthesis liquid and crystals after the synthesis.Meanwhile,a thermal aging treatment is designed for comparison.The mechanism of ultrasonic assisted synthesis is systematically discussed.The results show that the membrane synthesized at 24 h with 0.5 h ultrasonic treatment has a separation factor of over 10000 and a flux of 12.2 kg m-2h-1 for 90%isopropanol aqueous solution at 75 ?,while the membrane synthesized without ultrasonic treatment at 40 h has a separation factor of over 6000.When the treatment time is prolonged to 1 h,the separation performance of the membrane is not further improved.The only 0.5 h ultrasonic-assisted aging can effectively reduce the synthesis time from 40 h to 24 h.We believe that ultrasound has cavitation effect,can effectively promote the depolymerization and repolymerazation of silicon-aluminum sources and increase the amount of crystal nuclei in the solution,so dense membranes can be synthesized in a short period of time.Elongating the ultrasonic treatment time will not significantly increase the membrane performance further,which may be due to the competition between the growth of the crystal in the solution and the growth of the membrane on the support.When the porosity of the support is 68%,the flux of the membrane is as high as 14.3 kg m-2 h-1.Because the porosity is high and wall of?-Al2O3 hollow fiber is thin(-250 nm),the resistance of the support is reduced.That is why the flux of the membrane is improved.Compared with the ultrasonic aging,the effect of thermal aging treatment is weaker.(2)T-type zeolite membranes synthesized by hot water seeding on dopamine hydrochloride(DA)modified supportsDopamine hydrochloride(DA)is widely used in biosensors,materials,catalysts,and inorganic membrane preparation.In tris buffer solution,ploydopamine hydrochloride(PDA)membrane is formed,which can greatly improve the number of hydroxyl groups on the surface of the support,thus improve the connection between crystal and support.Here PDA modified ?-Al2O3 supports were used,and thein-situ synthesis of T-type membranes on PDA modified supports,and secondary growth of T-type zeolite membranes by hot water seeding on PDA modified supports and by removing PDA by calcination after hot water seeding were systematically explored.It is found that the surface roughness of PDA modified support is decreased.Therefore,the seed crystals could not be fixed directly by rub-coating method.The crystals can be fixed on the surface of the support through condensation reaction of hydroxyl on the surface of the PDA layer and the zeolite in hot water.Dense membranes can not be obtained by the in-situ synthesis on DA treated supports which is due to the surface defects of the support and the harsh synthesis conditions of T-type zeolite membranes.However,DA has a good promoting effect on the growth of T-type zeolite membranes.In the presence of DA,only a very small amount of seeds is required to prepare the dense membrane.In this paper,ultrasonic-assisted aging is used,the steps are simple and cost-effective.Meanwhile,the dopamine hydrochloride-assisted growth of T-type zeolite membranes is explored,which provides a new route for simplifying the crystallization process and further exploring the synthesis conditions of T-type zeolite membranes. |
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